1. Field of the Invention
This invention relates to method and apparatus for curing wood, and in particular to such method and apparatus which prevents condensation on the kiln floor, enhances the air flow around and through the stacked wood, uses swing-up doors to provide sealing reducing heat loss through condensation control and provides a constant negative pressure in the kiln to enable constant, slow venting to the ambient.
2. Related Art
In conventional wood kilns fans are used to force air through the stacked lumber within a sealed chamber, and baffles are required to ensure that the air is forced through the stacked lumber. In many conventional wood kilns such baffling is so insufficient that a considerable amount of the air is forced above the stacked lumber and not through it as is desired. One solution that has been adopted is to increase the amount of forced air flowing in the kiln, which is inefficient as it wastes electrical power and the heat rise caused by the fan blowers may actually result in a sufficient temperature increase within the kiln that it is difficult to maintain the proper temperature control during the wood curing cycle. Venting air to reduce the temperature may result in too dry air in the kiln and damage the wood. Vaporization normally provides natural cooling, but may be insufficient with certain types and thicknesses of wood such as 2 inch thick oak, for example. With such wood the rate of vaporization is too slow to provide adequate cooling to control the temperature of the air in the kiln.
Drying lumber differs from drying anything else due to the thickness of the wood. For example, if it is desired to dry wood down to an average of 5 or 6 percent moisture content, the wood is really 3 or 4 percent moisture content on the outside and 6 to 8 percent moisture content on the inside. The dry outside of the wood is in a condensed condition and the wet inside is in an expanded condition. If such wood were split, it would "pop" and be useless. Thus, it is necessary to equalize the lumber. The normal process for equalizing lumber is to dry it down to 3 percent and at this point the outside will be dry and the inside will be at about 6 percent. Most wood kilns release steam into the kiln and bring the humidity back up, which puts moisture back into the lumber that will equalize it to the same moisture content as the inside. The provision of steam for such equalization represents an additional cost of curing the lumber.
Another problem associated with conventional wood kilns is that the use of track-type doors causes sealing problems as the door never returns to the same position from which it started when it is opened and then closed as the seal has molded itself to the previous position the door was in when it was closed. As most conventional wood curing kilns operate with a positive pressure in the kiln it is necessary to have rather tight sealing around the kiln doors.
Most conventional wood kilns operate on a set schedule for drying the wood. Such schedules require two thermal elements to monitor the moisture and the temperature or heat in the kiln. Normally sample pieces of wood are inserted into the slits where the bolsters are in the stacked wood and these wood samples are then periodically weighed to determine the moisture content of the wood.
Finally, conventional wood kilns produce a great amount of condensation on the floor of the kiln and such condensation, because of the chemicals it contains, will seriously deteriorate the kiln floor. The periodic repair or replacement of the kiln floor is expensive and results in downtime of the kiln.
It is common practice in conventional wood kilns to equalize the moisture content of the stacked wood therein by introducing steam to accelerate the equalization process. While such use of steam does accelerate the equalization process, it also represents an added expense to the wood curing process.